unknow command - use the same error format as the Docker CLI

[glours]

- What I did Align Docker App error message to the Docker CLI when a command is unknown

- How I did it Remove usage message of the error output

- How to verify it

• execute the command line docker app, check it displays the usage message & check the return code is 0 with echo $?
• execute the command line with an invalid command such as docker app unknown_command, check it doesn't display the usage message & check the return code is 1 with echo $?

- A picture of a cute animal (not mandatory but encouraged)

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@695509b). Click here to learn what that means. The diff coverage is 0%.

@@            Coverage Diff            @@
##             master     #646   +/-   ##
=========================================
  Coverage          ?   71.94%           
=========================================
  Files             ?       49           
  Lines             ?     2545           
  Branches          ?        0           
=========================================
  Hits              ?     1831           
  Misses            ?      484           
  Partials          ?      230

Impacted Files	Coverage Δ
internal/commands/root.go	71.87% <0%> (ø)

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